This invention relates to precipitation processes for the remediation of acidic waste and drainage waters comprising metal and/or metalloid ions such as ions selected from (but not restricted to) the group consisting of copper, zinc, lead, mercury, cadmium, iron, arsenic, barium, selenium, silver, chromium, aluminium, manganese, nickel, cobalt, uranium and antimony.
In particular the invention relates to the remediation of acidic waste water comprising iron ions and sulfate ions, as well as other undesirable metal or metalloid species. Such waters are frequently formed as a result of the oxidation and leaching of sulfide minerals during and after mining operations, and are referred to as acid mine drainage.
Significant quantities of acid mine drainage are associated with many former and current mining operations such as the Berkeley Pit in Butte, Mont. USA. The problems caused by acid mine drainage include:
toxicity arising from particular metal contaminants e.g. cadmium, arsenic PA1 environmental pollution where the waters containing such metals are allowed to flow to areas producing contamination with toxic elements being labile or only loosely bound by adsorption PA1 the corrosive effects arising from the acidic pH values from sulfide oxidation to sulfuric acid PA1 the foregone opportunity of using the water values for irrigation or human purposes and PA1 the foregone opportunity of recovering useful metals contained in the contaminated water, e.g. copper and zinc. Approximately 8000 lbs (3600 Kilograms) of copper and 20,000 lbs (9000 Kilograms) zinc enter the Berkeley Pit each day. PA1 a) The limestone neutralisation/aeration step (pH 5-6) removes Fe, Al, Cu, and some sulfate. The Fe is removed as Fe(III) hydroxide (ferrihydrite), and whilst this precipitate has better settling characteristics than when lime is used as a precipitating agent, significant processing difficulties related to long settling times and filter clogging would be anticipated to occur. PA1 b) The lime neutralisation step removes cadmium, zinc and manganese from the aqueous phase, and a pH of 10 or greater is necessary to reduce manganese to below 3 ppm. The sludge formed has a large volume and is slimy and difficult to settle and filter. It is unstable with respect to discharge back into the Pit and must be disposed in a controlled area or extracted for metal values. PA1 c) Trace metals such as arsenic, cadmium etc are adsorbed on the surface of the ferrihydrite. This adsorption is charge-driven and if the pH changes to a value below the isoelectric point, adsorbed cationic material may be released back into the environment. PA1 d) The precipitate from the first limestone precipitation phase must be removed before the second lime precipitation reaction occurs, otherwise aluminium is re-solublised. This leads to the necessity of a double filtration step. PA1 e) Copper cannot be readily released from the limestone neutralisation sludge. PA1 f) The removal of sulfate ions by precipitation with barium oxide/hydroxide is expensive and the residual barium species are toxic. PA1 g) The air sparging steps and the pH polishing steps by carbon dioxide sparging are expensive in terms of pumping energy. PA1 the requirement for high temperatures leads to unacceptably high process costs, and, PA1 the requirement for a substantial excess of trivalent over divalent/monovalent metal species in the Jarosite structure is not compatible with the composition of acid mine drainage in many locations, e.g. the deep water of the Berkeley Pit where excess divalent species are present.
A wide variety of methods have been proposed for the remediation of acid waste drainage.
In-situ mitigation is a method whereby limestone placements are put down to collect surface run-off and funnel it into waste rock dumps. Such a method is described in SITE 94, p. 374, presented by the University of South Carolina. [The publication SITE 94 refers to the U.S. EPA Superfund Innovative Technology Evaluation Program, Technology Profiles, Seventh Edition 1994. This document is issued by the Risk Reduction Engineering Laboratory Office of Research and Development, U.S. EPA Cincinnati, Ohio, 45268, USA.] Acidic material is capped with an impermeable layer to divert water from the acid cores. This method relies on the existence of sufficient rainfall to produce seepage or drainage that continually contacts the limestone. The method has limited efficiency for remediation (as judged by the acidity of treated versus untreated areas) and is weather dependent.
Wet-lands based treatment has also been considered. This method uses a man-made wet-land ecosystem to remove heavy metals and is described in SITE 94, p. 164 (Colorado Department of Public Health and Environment). This method is not able to recover useful metals from the acid mine drainage and removal efficiencies are generally less than for chemical precipitation processes.
Another method described in SITE 94, p. 56 (Dynaphore Inc.) involves adsorption followed by pumping or drainage through an open-celled cellulose sponge containing amine-functional polymers with selective affinity for aqueous heavy metals in both cationic and anionic form. This method (in common with other methods based on absorbent or adsorbent material) has limited efficiency for the removal of some metal ions from a complex mixture and for anions such as sulfate. It is characterised by high capital costs and by the generation of relatively dilute strip liquor.
SITE 94 at p. 304 describes a method involving precipitation plus adsorption in which the pH of the waste stream is adjusted to 9-10 (under standard atmospheric conditions) followed by pumping/drainage through a column containing adsorbent ferrihydrite applied to the surface of an inert substrate such as sand. This method has limited efficiency for the removal of sulfate anions and generates relatively dilute strip liquor. For waste streams containing relatively high levels of ferrous ions such as acid mine drainage from the Berkeley Pit, the adjustment under oxidising atmospheric conditions to pH up to 9-10 will involve the precipitation of large quantities of amorphous or poorly crystalline ferrihydrite having particle size and surface characteristics which lead to high sludge volumes and consequently to facile blocking of separation columns.
Methods of simple precipitation by raising the pH to 9-11 using hydroxide anions under standard atmospheric conditions have been described. These methods lead to ferrihydrite formation and its attendant problem of high sludge volumes as described above. The requirement to go to pH 11 for some metals followed by neutralisation also adds significant costs and sulfate anions are not removed by this process. This method is described in Principles of Aquatic Chemistry, F. M. M. Morel, WileyInterscience Publication, John Wiley and Sons, 1993, and Aquatic Chemistry, W. Stumm and J. J Morgan, Wiley Interscience Publication, John Wiley and Sons, 1991. These references also describe simple precipitation using sulfide anions for the removal of many metal ions. This method requires post-treatment for the removal of sulfide in the effluent, and leads to the formation of toxic H.sub.2 S when the pH drops below 8.
Simple precipitation using carbonate anions is also described in these Wiley Interscience publications. Certain metal carbonates (cadmium, lead) can be precipitated at pH values in the range 7.5 to 8.5 as dense filterable sludges (whilst hydroxide precipitation of these elements occurs only at pH 10 or greater and leads to higher sludge volume). However the process is not effective for all metals, including zinc and nickel; iron(III) does not form a carbonate. Precipitation can be carried out using specialty agents such as RHM-1000 (SITE 94, p. 210, Technicon Environmental) or macromolecular complexing agents (SITE 94, p. 224 Atomic Energy of Canada Ltd). The use of these methods to treat vast quantities of acid mine drainage to exacting effluent standards is expensive.
Precipitation using an aqueous slurry of tailings from proximal mining operations has been suggested. This method is particularly appropriate for remediation of the Berkeley Pit site where alkaline tailings are produced at the Weed concentrator adjacent to the pit. The process is described in The Aqueous Geochemistry of the Berkeley Pit, Butte, Mont. USA. A. Davis and D. Ashenberg, Applied Geochemistry, Volume 4, pp 23-36, 1989. The optimal final state pH is believed to be 5, which optimises arsenic insolubility and provides a significant quantity of ferrihydrite (amorphous ferric oxide hydroxide), which in turn will partially bind trace metals in the sludge. Problems associated with the above method include the production of significant sludge volumes and the incomplete removal of some toxic metal contaminants.
Limestone neutralisation and aeration at pH 5-6 followed by a separate lime precipitation step (pH 9-10) followed by polishing with barium oxide/hydroxide for sulfate removal and neutralisation with carbon dioxide. This multi-stage process, which requires the removal of precipitated sludge in 2 or 3 separate operations, has been recommended for the remediation of acid mine drainage at the Berkeley Pit in The Chemical Precipitation Treatment Process for Acid Mine Drainage at the Berkeley Pit. Hsin--Hsiung Huang, Yibin Shi and Haiyang Gu, (Mine drainage Management and Remediation Conference, Fairmont Hot Springs, Opportunity, Mont. July 1992). Huang et al have also provided comparative data for a one-step lime neutralisation process. Features of the multi-step process include:
Huang et al. noted a number of problems with the neutralisation process including the inability to remove aluminium and manganese at the same time and the instability of the sludge with respect to re-dissolution in the Pit.
The removal of iron from acidic sulfate-containing leach liquors which have been used to process nonferrous metal ores (e.g. zinc, copper) has been achieved using the Jarosite precipitation process. (reference--Jarosites: A Review. GK Das et al. Min. Proc. Ext. Met. Rev., Vol 16, pp185-210 1996). Jarosite precipitates are formed at low pH values and elevated temperatures (60-100.degree. C.) under ambient atmospheric conditions in the presence of appropriate ions, e.g. barium calcium or lead. The formula for the Jarosite family of compounds is AB.sub.3 (XO.sub.4).sub.2 (OH).sub.n.mH.sub.2 O where A represents monovalent or divalent metal species (commonly Pb, possibly Ag, NH.sub.4, H.sub.3 O, Na, K) and B represents a trivalent or tetravalent metal species (e.g. Fe(III), Al(III), Sn(IV)). X represents a member of the family consisting of sulphur, phosphorus, silicon, arsenic. The most common Jarosites are based on trivalent iron and have a formula of M[Fe(OH).sub.2 ].sub.3 (SO.sub.4).sub.2 where M is the species H.sub.3 O, Li, Na, K, NH.sub.4, Ag and 0.5 Pb.
The Jarosite remediation process is not suitable for the remediation of acid mine drainage because